Re the mirror discussion, we had a near tragedy on yesterdays club ride when a cyclist failed to look behind before making a turn. She is new to the club, and was riding next to me in front of about 8 others on a rural 2 lane highway. We were about 10 minutes out of the last rest stop, with everyone fueled up and talking.
Anyway, I failed to spot our left turn in advance. A rider in back yelled "left turn, turn here". I could read the sign then, but just rolled past to stop on the side of the road, as we were just passing the intersection and going at a fair clip. To my surprise, the women next to me responded to the yell by hitting the brakes hard and then hooking the left turn, not realizing two cars were passing the group at the time, likely doing 55-60 mph. As soon as she turned, I heard the screach of tires from the cars. Fortunately, she cleared the lane a few tenths of a second before the long skid marks reached her. A couple of riders in the pack tangled and went down in the panic. They were shaken by the close call, but no one was seriously hurt.
Three points about safe riding habits from this episode. First, when in the lead (as I was) always stay alert for the next turn. Had I spotted the sign first, would have called it out, slowed, and looked back in my mirror for traffic to see if the group could complete the left turn safely.
Second, always look front and behind before turning left on a two lane road. With the 15 mph headwind we had, it was impossible to hear the cars coming up to pass at 60 mph. Believe a mirror really helps to make this a habit. But, if you don't use a mirror (she didn't), practice turning your head to get a good look while riding in a straight line.
Third point: never brake hard or unexpectedly with riders in back of you. Always use a hand signal, or call out slowing or stopping before taking action. It's easy to stop when past the turn and check for traffic before heading back onto the next road.
Sorry to lecture, but this near-miss on a beautiful Saturday afternoon reminded me how important it is to follow the basic rules of the road at all times. Also reminded me that little things like average speed and who's ahead or behind don't mean much; having all of your friends finish the ride safely is all that really matters.